Known methods for producing toluenediamines typically form these products as isomeric mixtures. Close boiling isomeric mixtures are separated by distillation to produce valuable products of which the most common and lowest cost product is a mixture of about 80% 2,4-toluenediamine (2,4-TDA), 19% 2,6-toluenediamine (2,6-TDA), and 1% 2,5-toluenediamine (2,5-TDA). Another chemical route yields mixture of about 65% 2,4-TDA, and about 35% 2,6-TDA. Further separation of these isomers by additional distillation has been shown to be impractical and non-viable financially.
2,4-TDA is useful as an intermediate for the synthesis of 3,5-dimethylthio-2,4-toluenediamine, which is a curative for TDI polyether and TDI polyester prepolymers.
Thus the need has arisen for an effective, commercially-feasible method for separating 2,4-TDA of suitably high purity from an isomeric mixture of toluenediamines, especially a mixture comprising at least 2,4-TDA and 2,6-TDA and in which from about 60 to about 85% of the mixture is 2,4-TDA, and from about 15 to about 40% of the mixture is 2,6-TDA. Such mixtures to be processed may also contain small amounts of 2,5-TDA, e.g., amounts of up to about 1%.
In the fulfillment of the foregoing need, this invention provides in one of its embodiments a process for separating 2,4-TDA of a purity of at least about 96 wt % from a mixture of toluenediamine isomers comprising at least 2,4-TDA and 2,6-TDA and in which from about 60 to about 85% of the mixture is 2,4-TDA and from about 15 to about 40% of the mixture is 2,6-TDA. The method comprises:
A) melting the isomer mixture, and then lowering the temperature of the mixture to a 2,4-toluenediamine nucleation temperature so that crystals plate out on a cooling surface;
B) gradually reducing the temperature of the mixture from the nucleation temperature to a temperature in the range of about 65 to about 70xc2x0 C. such that the mixture is in the form of (i) a solids phase enriched in 2,4-toluenediamine, at least a portion of the solids phase being attached to the cooling surface, and (ii) a liquid phase enriched in 2,6-toluenediamine; and
C) separating these phases from each other, preferably by draining from the solids attached to the cooling surface, the liquid phase enriched in 2,6-toluenediamine.
Before commencing the gradual reduction in temperature in B), it is preferred to maintain the isomer mixture formed in A) at the nucleation temperature for a period of time sufficient, but not substantially in excess of the time required, for the desired purity of the crystal lattice being formed to be achieved. Typically this period of time is somewhere in the range of about 0.25 to about 3 hours, depending upon the composition of the particular isomer mixture being processed, and the thermal conductivity and roughness of the surface on which the crystallization takes place. Shorter or longer time periods can be employed, however, whenever deemed necessary or desirable, and are within the scope of this invention.
In a preferred embodiment the time period in B) in which the temperature is gradually reduced from the nucleation temperature is in the range of about 6 to about 16 hours.
Another preferred embodiment of this invention is a method which enables maximization of 2,4-toluenediamine purity. This method comprises the following steps conducted seriatim:
a) melting the initial isomer mixture described above, and then lowering the temperature of the mixture to a 2,4-toluenediamine nucleation temperature so that crystals plate out on a cooling surface;
b) maintaining the mixture at the nucleation temperature for a period of time, which typically is in the range of about 0.25 to about 3 hours and preferably is in the range of about 0.75 to about 3 hours, that enables the 2,4-TDA solids remaining after conducting f) hereinafter to reach a purity of at least 96% and preferably of at least 98%;
c) gradually reducing the temperature of the mixture from the nucleation temperature to a temperature in the range of about 65 to about 70xc2x0 C. such that the mixture is in the form of (i) a solids phase enriched in 2,4-toluenediamine, at least a portion of the solids phase being attached to the cooling surface, and (ii) a liquid phase enriched in 2,6-toluenediamine;
d) separating these phases from each other, preferably by draining from the solids attached to the cooling surface, the liquid phase enriched in 2,6-toluenediamine;
e) heating slowly at least a portion of the solids phase remaining after the separation of d) so that impurities xe2x80x9csweatxe2x80x9d from the solids as liquids, and
f) separating these liquids or xe2x80x9csweatxe2x80x9d from the solids essentially as soon as these liquids or xe2x80x9csweatxe2x80x9d are being formed, preferably by continuously draining the liquids or xe2x80x9csweatxe2x80x9d away from the solids as soon they are formed.
Another step which is usually, but not necessarily, performed is to recover the solids remaining after completion of f). Preferably such recovery is accomplished by melting the solids remaining in the apparatus in which the overall operation is conducted (typically a melt crystallizer), and draining the melted solids from the apparatus. Instead of recovering these purified solids, it may be possible, depending upon the design of the apparatus, to react the solids while in the apparatus with a suitable reactant to form another desired product in situ, such as 3,5-di(methylthio)-2,4-toluenediamine, which is then recovered from the apparatus.
Typically the purified solids remaining after completion of f) contain about 96-99% 2,4-TDA, up to about 1% 2,5-TDA, with the balance, if any, being 2,6-TDA. Optimally, the composition of the xe2x80x9csweatxe2x80x9d is close to that of the initial feed to the process, e.g., ca. 80% 2,4-TDA and ca. 20% 2,6-TDA, or ca. 65% 2,4-TDA and ca. 35% 2,6-TDA.
Typically the liquid phase enriched in 2,6-toluenediamine formed in c) comprises about 36 to about 49% of 2,6-toluenediamine, optionally, up to about 1% of 2,5-toluenediamine, and wherein essentially all of the balance to 100% is 2,4-toluenediamine. In preferred compositions about 45 to about 49% of the composition is 2,6-toluenediamine. Such novel and useful compositions constitute still further embodiments of this invention.
If desired, the separated xe2x80x9csweatxe2x80x9d is recovered and recycled to a) together with another charge of a mixture of toluenediamine isomers comprising at least 2,4-TDA and 2,6-TDA.
Among the highly advantageous features of this invention is that a pair of highly useful products can be produced concurrently by the present process technology. First, the process enables the recovery in good yield of 2,4-TDA with a purity of 96-99%. So far as is presently known, no prior, commercially-practicable process has the capability of accomplishing this without substantially greater capital expenditure. Secondly, as described more fully in commonly-owned copending Application Ser. No. 09/528,837, filed Mar. 20, 2000, the liquid phase enriched in 2,6-toluenediamine formed in the present process, has been found to be an especially desirable starting material for the preparation of mixtures comprised of 3,5-dimethylthio-2,6-toluenediamine and 3,5-dimethylthio-2,4-toluenediamine. These latter mixtures when used as curatives with isocyanate prepolymers enable formation of polyurethane polymers having very desirable thermo-mechanical properties.
Still other embodiments and features of this invention will become still further apparent from the ensuing description and appended claims.